Systematic Evaluation of Materials to Enhance Soluble Phosphorus Removal Using Biofiltration or Bioswale Stormwater Management Controls | Journal of Sustainable Water in the Built Environment | Vol 9, No 1

AbstractSignificant environmental impact can be caused by surface runoff when it transports excessive phosphorus (P) to downstream surface waters. Excessive P can cause uncontrolled aquatic plant growth, low dissolved oxygen, and increased fish mortality. Urban stormwater is composed of both dissolved and particulate P fractions in roughly equal proportions. Many stormwater management controls (SMCs) like biofiltration and bioswales can remove particulate P easily through sedimentation and filtration processes, but dissolved P is much harder to remove. These SMCs also contain compost, which is excellent for removing many other pollutants but can decompose and release P. This laboratory study systematically evaluated dissolved P removal efficiency of 40 different adsorptive materials through jar tests and adsorption isotherm kinetics, column tests that emulate biofiltration systems, and channel tests to emulate bioinfiltration swales. The jar tests yielded 12 materials for further testing using complete adsorption isotherms. Seven of these 12 materials were tested extensively in the column studies. The best materials for enhancing P removal in the columns were activated alumina, alum-based water treatment residuals (WTRs), layered double hydroxyl (LDH), and zero-valent iron (ZVI) aggregate, all exceeding 90% P removal. Based on performance, three were chosen for the channel study: activated alumina, alum-based WTR, and ZVI aggregate. Results showed that lower compost content could reduce P leaching but not eliminate it completely, while biochar mixes were highly variable. Replacing half of the compost with an adsorptive material in a conventional bioretention/bioswale system gave good P removal results. The alum-based WTR is recommended in this study because it ranked excellently for all tests performed, and as a waste material, it can be recycled at a low cost.Practical ApplicationsHigh phosphorus concentrations in the surface waters act as a fertilizer, exacerbating the growth of aquatic plants and algae and harming the ecosystem. One source of phosphorus to surface waters is stormwater runoff from urban lawns and impervious surfaces. Treatment of stormwater using biofiltration media can reduce the problem if the treatment removes the dissolved phosphorus. In this project, 40 different adsorptive materials were tested for phosphorus removal and screened using batch adsorption experiments, column studies, and channel studies. This process identified an outstanding material, i.e., alum sludge, that works excellently and is essentially free. Alum sludge is the residual sludge material from drinking water treatment plants that use alum for turbidity removal. Biofiltration media with the addition of alum sludge can remove phosphorus up to 89%, copper up to 87%, and zinc up to 98%. The sludge also does not leach any aluminum. Globally, millions of tons of alum sludge are produced by water treatment plants daily. The addition of the alum sludge to the traditional biofiltration media is not only a cost-effective way of removing stormwater phosphorus but could also be a way of sustainably recycling huge volumes of water treatment plant residuals.

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